1. Field of the Invention
The invention relates to a method of treating an object of silicon steel having an insulating protective coating of silicate with a solution of phosphate.
2. The Prior Art
In the manufacture of sheet material of silicon steel, so-called electrical sheet, with grain orientation, the sheet material is subjected after rolling and decarburization to a heat treatment at about 850.degree. to 1350.degree.C in order to achieve a grain growth of the crystals which is necessary for the sheet material to acquire the required magnetic properties.
Before the heat treatment the sheet material is coated with chemicals which are to form an electrically insulating protective coating on the sheet material during the heat treatment process. One such known protective coating may consist of a reaction product of silicon dioxide formed at the surface of the sheet material and an applied oxide or hydroxide of an alkaline earth metal, usually magnesium. The application of the protective coating on the surface of the sheet material is carried out by suspending the alkaline earth metal oxide or hydroxide in water and applying it onto the sheet material in an even layer, after which the sheet is subjected to the previously mentioned heat treatment at a temperature of around 850.degree.C to 1350.degree.C in hydrogen atmosphere for several hours. However, in order to ensure that a well-developed glass film is formed on the sheet material the temperature should amount to around 1000.degree.C to 1350.degree.C. The hydroxide which is part of the suspension from the start or which is formed from the oxide by liberation of water liberates water while the sheet material is being heated, which water, at temperatures below that mentioned above, is able to oxidize silicon in the steel to silicon dioxide without simultaneously oxidizing the iron. The oxide which is formed from the hydroxide during the liberation of water or which is possibly added from the start and has escaped hydration, reacts with the silicon dioxide during the heat treatment while forming the previously mentioned well-developed glass film on the surface of the sheet material. The glass film can also be obtained by using a carbonate of an alkaline earth metal. The carbon dioxide which the carbonate liberates during heating is able to oxidize silicon into silicon dioxide without the iron being oxidized. When the silicon dioxide has been formed, the formation of glass proceeds in the manner described above. Any excess of the oxide which has not reacted during the glass formation acts as spacing material between adjacent layers of the sheet, whether these are present as turns in a roll or as laminae in a stack, and it prevents the layers from sticking or sintering together.
A protective coating of silicate as described above has an insulating resistance which is insufficient for many purposes, and therefore the protective coating is often reinforced by treatment with phosphoric acid and metal phosphates, especially alkaline earth metal phosphates, and the solution must be acid in order that the phosphates might be kept dissolved.
When applying phosphates on the said protective coating of silicate by known methods, the protective coating may become porous. This is due to the phosphate penetrating into the silicate layer and loosening it up. The penetration of the phosphate also causes the otherwise good adhesion of the coating to the sheet to deteriorate and also the tenacity of the coating and thus its resistance to, e.g., bending to deteriorate. The loosening of the protective coating also causes it to be easily broken away from the sheet during the subsequent annealing at around 800.degree.C, which is carried out in order to remove inner tensions in the sheet. In this operation gases may penetrate into the sheet material, owing to the porosity of the protective coating, thus causing the unfavourable effect.